1. Field of the Invention
The present invention relates to an air-to-fuel ratio feedback control system wherein the concentration of a constituent in the exhaust gases from an internal combustion engine is detected to thereby control the air-to-fuel ratio of mixture drawn into the engine.
2. Description of the Prior Art
It is known in the art to provide in the exhaust pipe of an internal combustion engine an air-to-fuel ratio detector to detect the concentration of oxygen in the engine exhaust gases and generate a detection signal whose output level changes at the stoichiometric air-to-fuel ratio, whereby a comparison circuit compares the detection signal with a predetermined reference value corresponding to the stoichiometric air-to-fuel ratio to determine whether the mixture supplied to the engine is richer or leaner than the stoichiometry and thereby feedback control the air-to-fuel ratio of the mixture supplied to the engine in accordance with the output signal of the comparison circuit. Another feedback control system of this type is also known in the art in which when there exists a feedback control stopping condition, such as, when the temperature of an engine is low as during starting periods of the engine or an air-to-fuel ratio detector is rendered inactive (inoperative) so that its output signal remains at a high voltage level as would be the case when the air-to-fuel ratio detector has detected that the air-to-fuel ratio is richer, the feedback control system for the air-to-fuel ratio is switched from the closed loop to an open loop to thereby stop the feedback control. A disadvantage of this control method is that since the fact that the output signal of the air-to-fuel ratio detector has been remaining higher than a preset value of the comparison circuit is detected as a criterion for determining the necessity to effect switching between the open and closed loops of the feedback control system and since this preset value of the comparison circuit is the same with the preset value for determining whether the air-to-fuel ratio is rich or lean during the closed loop control, during transient periods such as during periods of transition from the open loop to the closed loop or from the closed loop to the open loop, there is the danger of the feedback control loop being intermittently switched from the open loop to the closed loop and vice versa by electrical noise induced on the output signal of the air-to-fuel ratio detector.